Amplifiers of class B or AB, in which at least the final stage draws current only during part of a signal cycle, have the advantage of greater output power in comparison with class-A amplifiers, enabling transistors of limited physical dimensions to produce large-amplitude output signals. This is particularly important for audio amplifiers realized with integrated circuitry.
In a larger system, in which such an amplifier is energized along with one or more additional amplifiers or other equipment from a common direct-current source, inconveniences arise from the fact that the current consumption of the amplifier varies with signal amplitude; this variation, of course, influences the operation of the remaining device or devices drawing current from the common source. A conventional solution of this problem (aside from the possibility of using a separate power supply for any amplifier of the class referred to) lies in the provision of a current sink connected in parallel with the amplifier of class B or AB and controlled to draw direct current at a rate complementary to that of the amplifier. This solution, evidently, is rather uneconomical in view of the wasteful energy dissipation in the current sink.